Development utilizing electric fields

ABSTRACT

To control toner used in developing a latent electrostatic image carried by, say, a photoconductive surface, there is a shaped electric field acting on a toner laden donor member for separating toner from the donor member as it enters a development zone and for redepositing any excess toner on the donor member as it leaves the development zone.

I United States Patent 1191 1111 3,914,460 Maksymiak Oct. 21, 1975 1 1DEVELOPMENT UTILIZING ELECTRIC 3,232,190 2/1966 Willmott 117/175 FIELDS3,257,223 6/1966 King l17/l7.5 3,306,193 2/1967 Rarey et al. 117/175 e oJ y a P -Y- 3,331,355 7/1967 Donalies et al. 118/637 3,592,166 7/1971 Wu118/637 [73] Ass'gnee' Stamford 3,645,770 2/1972 Flint 96/1 s1) Com3,696,783 10 1972 Fantuzzo 118/637 22 Fil d; J 9 1973 3,696,785 10/1972Andrus 355/3 R 3,707,389 12/1972 Maksymiak et a1 1 18/637 [21] Appl.No.: 322,254 3,739,748 6/1973 Rittler et a1 118/637 52 U.S. c1. 427/14;96/1 SD; 118/637; pimary sofoclews 355/3 R 51 Int. Cl. G03g 13/08; G03g15/08 [57] ABSTRACT [58] Field of Search 117/175; 118/637; To Controltoner used in developmg a latent electro- 355/3 R 3 119 SD static imagecarried by, say, a photoconductive surface, there is a shaped electricfield acting on a toner 5 References Cited laden donor member forseparating toner from the UNITED STATES PATENTS donor member as itenters a development zone and for 2 901 374 8/1959 G (H h l 17 17 5redepositing any excess toner on the donor member as 310401704 6/1962118 /63 7 it leaves the development Zone 3,203,394 8/1965 Hope et al.117/175 9 Claims, 6 Drawing Figures US. Patent Oct.21, 1975 Sheetlof2"3,914,460

U.S. Patent Oct. 21, 1975 Shet 2 of 2 DEVELOPMENT UTILIZING ELECTRICFIELDS BACKGROUND OF THE INVENTION This invention relates to developinglatent images with toner and, more particularly, to a development methodand apparatus utilizing a toner-laden donor member to apply tonerparticles to a surface bearing an electrostatic latent image thereon.

In reproduction processes, such as in xerography, wherein a latent imageis first formed on a surface and then made visible, or developed, with apowderous material commonly called toner, it is necessary to provide ameans for bringing the toner material adjacent the surface so that itcan be attracted to the surface in a pattern conforming to the latentimage thereon. One manner in which this is carried out is by theemployment of a donor member. The donor member, which can take the shapeof a roller, endless belt or other convenient form, is first loaded withtoner and then brought into close proximity to the surface beingdeveloped. The toner is attracted from the donor member onto the surfacein conformance with the electrostatic latent image thereby creating avisible image.

A number of measures can be taken to improve the operation of a donormember used in this development mode. For instance, if the toner layeris spaced from the surface being developed so that it does not touch it,there is a less likelihood that toner supplied by the donor member willdeposit in the non-image areas. This is because the toner on the donormember does not touch the surface and the non-image areas will notattract the toner.

This technique is described in US. Pat. No. 3,232,190 wherein a thinlayer of toner is positively held and maintained in a spaced relation tothe surface being developed. The high potential areas of the chargepattern exert electrostatic forces of sufficient intensity to overcomethe forces holding the toner on the donor member and these tonerparticles are attracted to image areas. However, the electrostaticforces exerted by the remaining areas of the surface, the non-imageareas, are insufficient to overcome forces attracting the adjacent tonerto the donor member and, consequently, the non-image areas remainrelatively free of toner.

The density of the final copy can be further controlled by selecting thegap distance between the donor member and surface. Since the strength ofthe electrostatic fields terminating on the surface being developeddecreases as a function of the distance from that surface, the gapdistance between the toner layer on the donor member and the surface isvery critical. Even at relatively modest gap distances it has been foundthat less than complete development takes place. Development has beenenhanced by placing a potential on the donor member itself which tendsto aid the toner in overcoming the forces attracting it to the donormember, but, when such a potential is applied, the toner tends to bedriven to the surface in random fashion and deposited in non-imageareas. This condition is known as background development and degradesthe quality of the final copy.

Another known method for releasing the toner from the donor member isdescribed in US. Pat. No. 3,257,223. This patent describes the creationof a cloud of toner particles between the donor member and the surfacebeing developed. The donor member is a three-layer structure including aconductive base, a dielectric layer over the base and a conductive gridon top of the dielectric layer.

During loading of the donor member, a potential opposite in polarity toa charge on the toner particles is placed on the conductive base therebyattracting toner particles to the surface areas between the gridelements. Then, when development is to take place, the potential isremoved from the conductive base, or, alternatively, a potential of thesame polarity as the charge on the toner particles is placed on theconductive base. Both create a condition which tends to blow the toneroff the donor member in a cloud and into contact with the surface beingdeveloped. The overlying conductive grid serves generally to breakup thefield lines emanating from the conductive backing of the donor duringloading and blow-off so as to create the characteristically strongfringe fields which exist over a surface having variable electricalpotential. Because the toner is in the form of a cloud, the possibilityof random deposition of toner, particularly in nonimage areas, isincreased.

A third type of donor member also involves creating fringe fields andthis is described in US. Pat. No. 3,203,394. The donor member has aconductive base with a poorly insulating layer thereon and then apattern of good insulating posts as a top layer. This donorconfiguration is used basically to make it easier to load toner on theperiphery of the donor prior to bringing it into the development zone.In operation, the structure is electrostatically charged and thenbrought into the loading area. Between the charging station and theloading station, the poorly insulating layer loses its charge so thatthe fringe fields are created on the surface between the insulatingposts and the poorly insulating intermediate layer.

As a result of the fringe fields, toner is placed only between theinsulating posts thereby creating a discontinuous layer of toner on thedonor. This is not totally desirable because it limits the amount oftoner that can be brought to the development zone. In addition, theposition of the piles of toner on the donor relative to the image areason the surface may not always be in synchronism. If this is the case,certain of the image areas on the surface would not be fully developedfor lack of toner.

The present invention relates to a development method and apparatuswhich imposes electrical fields on the toner layer and donor memberwithin the development zone so that toner is made readily available toimage areas of the surface being developed yet is not randomly depositedon the non-image areas. In one embodiment, the donor member takes theform of an endless rotatable belt passing through a toner loading areaand then adjacent the surface being developed. Toner loaded onto thebelt is subjected to an electrical field which activates the toner onthe donor member as it passes through the development zone. Those tonerparticles are attracted to image areas of the surface during activationfollow the field back to the donor surface where they are removed fromthe development zone. In the development zone, although toner isloosened and- /or separated from the donor due to the electrical field,it continues to be under the control of the electrical field. Thisarrangement prevents random deposition of toner in the non-image areasthereby reducing background. This arrangement also allows greaterflexibility in establishing gap distance between donor and surfacewithout the usual drop off in development quality.

Accordingly, it is an object of the invention to provide an improvedmethod and apparatus for developing electrostatic latent images.

It is a further object of the invention to provide an improved methodand apparatus for providing toner to a surface being developed by adonor member.

It is a further object of the invention to provide an improved methodand apparatus for providing toner to the surface being developed whenthe toner donor member is spaced from the surface.

It is a further object of the invention to provide an improved methodand apparatus for the control of toner particles in the developmentzone.

It is a further object of the invention to provide a development systemwherein little or no background is produced.

SUMMARY The present invention relates to a method and apparatus fordeveloping electrostatic latent images by bringing a toner-laden donormember adjacent a surface bearing a latent image and thereupontransferring the toner to the surface in conformance with the image. Thetoner is exposed to an electric field in the development zone whichactivates it as it passes into the development zone thereby making itreadily-attractable by the electrostatic image on the surface. Theelectric field is shaped so that the activated toner which is notattracted to image areas of the surface is redeposited on the donormember as it leaves the development zone.

In one embodiment, the toner layer on the donor is separated from thesurface by a gap and the electric field is sufficient to reduce theattraction of the toner to the donor or, in the alternative, separatethe toner from the donor. In a second embodiment, the toner layer can bein contact with the surface being developed and the affect of the fieldis to control the action of the toner in the development zone so thatlittle or no toner is deposited in non-image areas of the surface.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of theinvention as well as other objects and further features thereofreference is had to the following detailed description of the invention,which is to be read in conjunction with the accompanying drawingswherein:

FIG. 1 is a schematic illustration of the invention in a xerographicsystem.

FIG. 2 is a detailed view of an embodiment of the invention.

FIGS. 3 and 4 are detailed views of the electric field in thedevelopment zone.

FIG. 5 is a detailed view of another embodiment of .the invention.

FIG. 6 is another detailed view of the electric field in the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention can beadapted to any reproduction apparatus wherein toner is used to developan electrostatic latent image. For the purpose of this disclosure,however, the invention will be described principally within theenvironment of a xerographic reproduction apparatus.

Referring to FIG. 1, there is shown a schematic view of a continuousxerographic copier having a photosensitive member in the shape of a drum1] on which a latent electrostatic image of the information to bereproduced is formed. The rotatable drum 1] is driven about shaft 2 byany suitable means so that the drum surface passes through stations AEin a continuous manner. The peripheral surface of the drum is covered byan electrically conductive material which, in turn, is covered on itsouter surface with a photoconductive material such as vitreous selenium.The drum has five processing stations located about its periphery whichcarry out the steps of the xerographic process. These stations includecharging station A, exposing station B, developing station C, transferstation D, and cleaning station E.

An electrostatic latent image is formed on the drum 1] by passing itssurface through charging station A and exposure station B. Chargingstation A can include any suitable means for placing a uniform charge onthe photoconductive layer such as a corona charging device. Exposingstation B can include any suitable device which projects and focuses alight pattern on the drum conforming to the image to be reproduced bythe xerographic system. The light image projected onto the chargedphotoconductive layer of the drum is synchronized with the movement ofthe drum and causes selective charge dissipation on elemental areas ofthe drum to form an electrostatic latent image thereon.

After the formation of the electrostatic latent image by passing thedrum through stations A and B, the drum carries the latent image throughdeveloping station C. Within the developing station, a suitabledeveloper material (hereinafter referred to a toner), such as apigmented resinous electroscopic powder, is deposited on the drum inimagewise configuration according to the teachings of this invention. Amore detailed description of the development station follows.

After the development step, the drum carries the toner image throughtransfer station D where the toner image is transferred from the drumsurface to any suitable support material such as a sheet of paper,continuous web, or any other form of substrate which can be used toreceive toner images. Transfer of the toner image onto the copy sheet iscarried out in any suitable manner such as by electrostaticallyattracting the toner image from the drum onto a sheet of paper with acorona discharge device.

After the toner image has been transferred to the copy sheet and thecopy sheet has passed through the transfer station, a fixing device orother suitable processing station fixes the toner image. Any suitablefixing device which makes the toner permanent on the copy sheet issuitable.

The final station shown is cleaning station E. Station E can include anysuitable cleaning device, such as fur brush, which contacts thephotoconductive surface of the drum. The cleaning station is utilized toremove any residue toner particles from the photosensitive surface FIG.1, toner 18 picked up'from reservoir 17 by donor member 12 is broughtadjacent the drum 11 at station C. The donor member is shown in the formof an endless belt and rotates in the clockwise direction. The tonerparticles adhere to the belt and pass around roller 13 and fieldproducing device 3 where they are transferred to the drum to develop theimage thereon. As the belt transports the toner towards the developmentzone, the toner can be sprayed electrostatically by corona chargingdevice 16 to assure that all toner particles have the same charge.

FIG. 2 provides a more detailed view of the donor member. The belt 12may be insulating and may be made of a material which when rubbed withtoner produces a triboelectric action making the toner attractive to it.However, loading of the belt may be carried out without regard to itstriboelectric relationship to the toner. In such a case, loading may beaided by grounding roller 13 and vibrating reservoir 17 by any suitablemeans.

A number of different loading systems could be carried to load toner onthe donor member. For instance, the donor member could be made of amaterial having a suitable triboelectric relationship to toner particlesso that toner could be cascaded along with carrier beads over thesurface of the donor, the toner adhering to the donor member intraditional cascade development fashion. Another known way of placingtoner on a donor member would be to expose the member to a charged cloudof toner particles which would be attracted to the donor member surface.

After loading toner on the belt for transport to the development stationthe toner layer 19 may optionally be sprayed with a uniform charge suchas by corona device 16. This provides good attraction between the tonerand image areas of the drum during development. The charge sprayed onthe toner layer will be selected based on the development system used.For instance, it is common to charge a xerographic to a high positivepotential and then expose the non-image areas to light which reduces thepotential on those portions of the drum. In this case, if the highpotential areas are to be developed, the toner would be chargednegatively. However, if the low potential areas were to be developed,the toner would be charged positively.

Field producing apparatus 3 includes ground plate and biased plate 14.Ground plate 15 is placed behind belt 12 and forms the surface uponwhich belt 12 rides. Plate 15 may extend from the development zonethrough the charging area as shown. The other portion of field-producingapparatus 3 is bias plate 14 which can also form the surface upon whichthe belt rides as it exits the development zone. Plates l4 and 15 arenot in electrical contact because it is the combination of these twomembers which creates the electrical field through the development zoneto be described in more detail in FIGS. 3 and 4. In FIG. 1 there isprovided an air space 30 between members 14 and 15 for electricinsulation while in FIG. 2 insulating plate 31 provides properinsulation between the plates.

The potential applied to plate 14 will depend on the charge of the tonerparticles and is opposite thereto in polarity. As described in FIG. 2,if a negative charge is sprayed on to the toner layer by corona device16, the bias on plate 14 will be positive. On the other hand, if thepositive toner is used, plate 14 will be biased to a negative polarity.The combination of biased plate 14 and grounded plate 15 generate anelectric field between the two plates and toner in the development zonewill be attracted towards plate 14.

FIGS. 3 and 4 show a more detailed view of the development zone. Due tothe fact that member 15 is grounded and member 14 is at a positivepotential, a fringe field effect will be created between these twomembers in the vicinity of the development zone. This electric fieldextends through donor member 12 and toner layer 19. The lines of forcein the fringe field is indicated by broken lines passing between members14 and 15.

The purpose of the field is to activate the toner while it is in thedevelopment zone so that it is readily attractable by the electrostaticimage on the drum. The term activation as used herein includes, in thealternative, reducing the forces that attract the toner to the donor orreversing the forces to separate the toner from the donor. Inout-of-contact systems wherein the toner layer is separated from thedrum by a gap, as shown in FIGS. 3 and 4, the electric field can be ofsuch force as to actually dislodge and separate toner particles from thebelt in the development zone. In other systems, such as wherein thetoner layer is in contact with the drum, the attraction of the toner tothe donor is merely lessened or reduced to zero so that the image areasof the drum can attract it to the drum.

In out-of-contact systems, it is desirable to separate the tonerparticles from the donor sufficiently so that the particles are easilyattractable to the image areas on the surface being developed. However,it is equally important not to lose control of these activated tonerparticles since this encourages spurious deposition on the non-imageareas of the drum.

In FIG. 3, the pluses indicate the image areas of the drum, the balanceof the drum being the non-image areas. Toner which is activated byfollowing the electric field created by plates 14 and 15 is free to beattracted to the image areas. However, since an image area is notadjacent the donor at the moment shown, the activated toner does notcross the gap to the surface being developed. Instead, the tonerparticles complete their journey in conformance with the electric fieldgenerated by plates 14 and 15 and are reattracted onto the donormemberdue to the electrical field produced by member 14.

Referring to FIG. 4, there is shown the condition when an image area ismore clearly in the development zone and is available for the receipt oftoner from the donor member. In this case, the fields created by theimage areas of the drum attract activated toner as needed across the gapand onto the drum surface to develop the image. Any activated toner notso attracted completes its journey through the development zone inconformance with the electric field created by the donor member and isredeposited onto the surface of member 15.

As can be seen from the above description, although all toner on thedevelopment member may be activated, there is complete control over themovement of this toner at all times. This control operates to preventdeposition of toner on the non-image areas which is quite usual in thosesystems wherein blow-off fields are used to drive the toner across thegap onto the surface being developed. In addition, all toner on thedonor is subject to activation since it must pass the field producingmeans. This affords the fullest opportunity for complete development ofthe drum even in out-of-contact systems.

In addition to preventing the random deposition of toner in backgroundareas, the present invention also allows a good deal of flexibility inthe spacing of the donor member from the surface being developed. Oneproblem in previously known apparatus has been that it was notalwayspossible to have sufficiently strong fields from the image areasof the drum to reliably remove toner from the donor member at larger gapdistances. In the present invention, gap spacing is not as critical andlarger spacing is readily permitted. This is because the electric fieldwhich activates the toner frees the toner from its adhesion to the donormember, thereby reducing the electrostatic force required to attracttoner to the drum.

FIG. discloses another embodiment of the invention. Here an endless beltsuch as insulating belt 20 is supported and driven about rollers 21 and22 in a counterclockwise manner. The external surface of the belt picksup toner particles 7 from vibrating toner reservoir 26. To aid thedeposition of toner on the external surface of belt 20, a groundedelectrode 23 is placed over the hopper 26 and extends toward thedevelopment zone. Toner particles 28 picked up by the belt are broughtinto the development zone of drum 29 by the moving belt. In thedevelopment zone and behind the donor belt 20, a structure similar tothat of the embodiment in FIG. 2 is shown. Plate 23 which iselectrically grounded is also electrically insulated from plate 25 byinsulating material 24. Plate 25 has a polarity opposite that of thetoner particles.

As can be seen from FIG. 6, the electric field generated in thedevelopment zone across plates 23 and 25 is similar to that described inconjunction with FIGS. 3 and 4. When image areas in FIG. 6 pass throughthe development zone, activated toner particles which are following theelectric field between plates 23 and 25 are free to be attracted to theimage areas on drum 11. Any activated toner particles which are notattracted to the drum are deposited onto the belt as it exits thedevelopment zone, and are not left in an uncontrolled situation withinthe development zone.

It should be understood that a number of alternatives to the system areavailable which would fall within the scope of the invention. Forinstance, the charge on the toner particles delivered by the donormember can be positive or negative depending upon the nature of thecharge in the image areas. The configuration of the donor member is notnecessarily limited to an endless belt. For instance, the donor couldtake to the shape of a roller or a continuous web. Also, in an endlessbelt arrangement, it may be desirable to remove the residue toner fromthe belt before it is reloaded with toner to avoid history effects. Thiscan be carried out by any suitable cleaning device such as brush 4 shownin FIG. 2.

The developing method and apparatus disclosed can be used as part of anylarger reproduction apparatus where it is desired to develop anelectrostatic latent image. Thus, in addition to xerographic systems,other electrostatographic imaging systems can utilize this processincluding other electrophotographic and electrography systems.

In addition to the apparatus outlined, many modifications and/oradditions to this invention will be readily apparent to those skilled inthe art upon reading this disclosure and these are intended to beencompassed within the invention disclosed and claimed herein.

What is claimed is: l. A method of developing an electrostatic latentimage carried by a surface, said method comprising the steps of:

bringing a toner-laden donor member adjacent said surface in adevelopment zone, and

subjecting the toner to an electric field shaped to firstseparate'toner' from the donor member as the toner enters thedevelopment zone and to then redeposit toner not attracted to thesurface onto the donor member as the toner leaves the development zone,whereby the toner in said development zone is controlled but stillreadily'attractable to said surface in conformance with the imagethereon.

2. The method according to claim 1 further including a preliminary stepof loading toner on the donor member.

3. The method according to claim 1 further including a preliminary stepof charging the toner to a given polarity prior to its passing throughthe development zone.

4. The method according to claim 1 wherein all the toner on the donormember is subjected to the electric field as it passes through thedevelopment zone.

5. A method of developing an electrostatic latent image carried by asurface as said surface movesthrough a development zone, said methodcomprising the steps of:

transporting toner to the development zone by a web,

and

subjecting the toner and web to an electric field shaped to separatetoner from the web as the toner enters the development zone and toredeposit the toner not attracted to the surface onto the web as thetoner leaves the development zone, whereby toner is available fordevelopment of said image and excess toner is removed by said web fromsaid development zone.

6. The method according to claim 5 wherein the web is an endless beltadapted to continuously pass through the development zone and furtherincluding the preliminary step of loading toner onto the belt.

7. The method according to claim 6 further including the additional stepof .cleaning unused toner from the belt.

8. In a method of developing an electrostatic latent image carried by asurface, said method including the step of bringing a toner-laden donormember adjacent said surface in a development zone; the improvementcomprising the additional step of:

subjecting the toner to an electric field shaped to reduce theattraction of the toner to the donor member as the toner enters thedevelopment zone whereby the toner is readily-attractable to the surfacein conformance with the image thereon and to increase the attraction ofthe toner to the donor member as said donor member leaves thedevelopment zone whereby toner not attracted to said surface is removedfrom said development zone.

9. In a method of developing an electrostatic latent ima'ge carried by asurface, said method including the step of transporting toner to thedevelopment zone by a web; the improvement comprising the additionalstep of subjecting the toner and web to an electric field whichactivates the toner as it enters the development zone whereby the toneris readily-attractable to the surface in conformance with the imagethereon and which reattracts the activated toner not attracted to thesurface onto the web as it leaves the development zone.

1. IN METHOD OF DEVELOPING AN ELECTROSTATIC LATENT IMAGE CARRIED BY A SURFACE, SAID METHOD COMPRISING THE STEPS OF: BRINGING A TONER-LADEN DONOR MEMBER ADJACENT SAID SURFACE IN A DEVELOPMENT ZONE, AND SUBJECTING THE TONER TO AN ELECTRIC FIELD SHAPED TO FIRST SEPARATE TONER THE DONOR MEMBER AS THE TONER ENTERS THE DEVELOPMENT ZONE AND TO THEN REDEPOSIT TONER NOT ATTRACTED TO THE SURFACE ONTO THE DONOR MEMBER AS THE TONER LEAVES THE DEVELOPMENT ZONE, WHEREBY THE TONER IN SAID DEVELOPMENT ZONE IS CONTROLLED BUT STILL READILY ATTRACTABLE TO SAID SURFACE IN CONFORMANCE WITH THE IMAGE THEREON.
 2. The method according to claim 1 further including a preliminary step of loading toner on the donor member.
 3. The method according to claim 1 further including a preliminary step of charging the toner to a given polarity prior to its passing through the development zone.
 4. The method according to claim 1 wherein all the toner on the donor member is subjected to the electric field as it passes through the development zone.
 5. A method of developing an electrostatic latent image carried by a surface as said surface moves through a development zone, said method comprising the steps of: transporting toner to the development zone by a web, and subjecting the toner and web to an electric field shaped to separate toner from the web as the toner enters the development zone and to redeposit the toner not attracted to the surface onto the web as the toner leaves the development zone, whereby toner is available for development of said image and excess toner is removed by said web from said development zone.
 6. The method according to claim 5 wherein the web is an endless belt adapted to continuously pass through the development zone and further including the preliminary step of loading toner onto the belt.
 7. The method according to claim 6 further including the additional step of cleaning unused toner from the belt.
 8. In a method of developing an electrostatic latent image carried by a surface, said method including the step of bringing a toner-laden donor member adjacent said surface in a development zone; the improvement comprising the additional step of: subjecting the toner to an electric field shaped to reduce the attraction of the toner to the donor member as the toner enters the development zone whereby the toner is readily-attractable to the surface in conformance with the image thereon and to increase the attraction of the toner to the donor member as said donor member leaves the development zone whereby toner not attracted to said surface is removed from said development zone.
 9. In a method of developing an electrostatic latent image carried by a surface, said method including the step of transporting toner to the development zone by a web; the improvement comprising the additional step of subjecting the toner and web to an electric field which activates the toner as it enters the development zone whereby the toner is readily-attractable to the surface In conformance with the image thereon and which reattracts the activated toner not attracted to the surface onto the web as it leaves the development zone. 